1. Field of the Invention
The invention relates to optical disk drives, and more particularly to layer changes for optical disk drives.
2. Description of the Related Art
Current optical disks have higher and higher data densities to store greater amounts of data in a single disk. Some optical disks, such as dual-layer Digital Versatile Disks (DVD) and Blu-ray Disks (BD), have multiple data layers for data storage. Referring to FIG. 1, a schematic diagram of an optical disk drive 100 is shown. The optical disk drive 100 has a pickup head 110 to access data of a disk 120. The disk 120 has a cover layer 126 and two data layers 122 and 124. To access data recorded on a target data layer selected from the data layers 122 and 124 of the disk 120, the optical disk drive must focus a laserbeam 106 emitted by the pickup head 110 on the target data layer to obtain a reflection signal reflected from the target data layer, and the reflection signal is then processed to obtain the data recorded on the target data layer.
When the laserbeam 106 is focused on a current data layer but the optical disk drive 100 requires accessing a target data layer different from the current data layer, the optical disk drive must perform a layer change procedure to focus the laserbeam on the target data layer. To carry out the layer change procedure, a spherical aberration correction process and a focus search process are performed. First, the optical disk drive 100 must trigger a stepping motor to move a collimator lens 102 of the pickup head 110, thus adjusting a relative distance between an objective lens 104 and the collimator lens 102 to perform a spherical aberration correction. In addition, the optical disk drive 100 must trigger a focus coil to alter the vertical position of the pickup head 110 until a focus of the laserbeam 106 is projected on the target data layer, referred to as a focus search procedure.
Conventional methods for controlling layer changes have shortages. First, when an optical disk drive 100 performs a focus search process, the optical disk drive 100 determines whether a focus of the laserbeam 106 is projected on a target data layer by detecting whether an S-curve is present in a focus error signal when the vertical position of the objective lens 104 is adjusted. If an S-curve is present, the focus is projected directly on a target data layer and the vertical position of the objective lens 104 is fixed to the current position. Otherwise, if no S-curve is detected, the objective lens 104 keeps searching until an S-curve is detected, and the objective lens 104 may collide with the surface of the optical disk 120.
In addition, when an optical disk 120 is clamped on a tray of an optical disk drive 100, the surface of the optical disk 120 is not always perpendicular to a spindle axis of the optical disk drive 100. If the surface of the optical disk 120 is not perpendicular to the spindle axis, the surface of the optical disk 120 periodically moves up and down with a revolution angle thereof when the optical disk 120 is spun by a spindle motor. Thus, there is a vertical relative motion between a pickup head of the optical disk drive and the disk surface moving up and down. The relative motion distorts the focus error signal, thus negatively affecting determination of an S-curve and decreasing correctness of a focus search process of the layer change procedure.
Furthermore, because layer changes are frequently performed to access an optical disk, efficiency of the layer change procedure greatly affects performance of the optical disk drive. Because the spherical aberration correction process require longer amounts of time, the time starting the focus search process altering the vertical position of the objective lens 104 must be carefully adjusted according to the timings of the spherical aberration correction procedure, thereby reducing the entire time required by the layer change procedure. Thus, a method for controlling layer changes for an optical disk drive is required.